vbe: Update simple-fw to support using the SPL loader

vbe: Update simple-fw to support using the SPL loader

For a sandbox implementation, where code size is no object, it makes sense
to use the full bootstd drivers to load images.

For real boards, running from SRAM, this adds quite a bit of overhead.

Add a way to load the next phase using just the underlying storage
driver, to reduce code size. For now, only MMC is supported.

Change the log_debug() to show the load address and size in a more
neutral way, rather than suggesting that the load has already happened.

Signed-off-by: Simon Glass <sjg@chromium.org>